Engineering Wonders of the World
Volume III

CONSTRUCTION OF AEROPLANES AND PROPELLERS. 43 A PROPELLER WHIRLING AT HIGH SPEED. {Photo, Illustrations Bureau.) be 1,500 feet per minute. High velocity of slip is not necessarily a test of thrust, as it depends largely on the resistance of the machine to the air. In practice it is found that a large propeller turn- ing at comparatively low speeds gives a greater thrust than a smaller propeller driven at very high speed, the power exerted being the same in both cases, and the pitch proportioned to give the requisite flight speed ne- cessary to support the aero- plane. For this reason the Wrights use two large slow- cient, the full thrust for power may be obtained either by using a small screw revolving at engine speed, or a larger screw turning at less than engine speed. In the first case the mass of air is less than in the second case, but the velocity imparted to it is greater ; in the second, the mass is larger but the velocity less. The essential point is to pro- portion and gear the propeller so that the engine shall be able to run at its most efficient speed. So far the imparting of motion to air by a fixed propeller has been considered. To obtain the rate of progression in feet per minute at which a machine would be Slip. driven by the propeller through the air one must multiply the pitch of the propeller in feet by the number of revolutions per minute, and deduct the “ slip ”—that is, the velocity of the air flung back by the pro- peller. A propeller with a 5-foot pitch re- volving four hundred times per minute would have a “ designed ” forward speed of 2,000 feet per minute. If the air left it at 500 feet per minute, th© actual speed of the machine would speed propellers, to which is due, in no small degree, the high efficiency of their machines proportionately to the horse- power of the motors employed. Convenience of attachment is a point in favour of the direct driven propeller, found on most monoplanes and many biplanes. There is a growing tend- ency, however, to increase the size of the pro- peller where convenient. We may note, by way of example, that Blériot now uses geared- down screws of large diameter for his heaviest monoplanes. The highest efficiency obtained so far by an aerial propeller does not exceed probably 70 per cent. It is anticipated that this may be improved upon until 85 to 90 per cent, of the engine power is usefully applied. This will make possible a considerable reduction in weight of engine, which in turn will lead to a diminution in the size of aeroplanes. Propellers are made of steel, aluminium, magnalium, and various kinds of wood. On the whole, th© Construction . of Propellers, wooden propeller appears to be most satisfactory. It can be made ex- ceedingly light without sacrificing strength,